scope:

The present document contains a detailed description of the handover procedures to be used in GSM PLMNs. The purpose of the handover procedures, as described in the present document, are to ensure that the connection to the Mobile Station (MS) is maintained as it moves from one Base Station System (BSS) area to another.

The present document considers the following two cases:

i) Handover between Base Stations connected to the same MSC, this is termed an Intra-MSC handover.

ii) Handover between Base Stations connected to different MSCs, this is termed an Inter-MSC handover. This category can be sub-divided into three further procedures:

a) the Basic Inter-MSC Handover procedure, where the MS is handed over from a controlling MSC (MSC-A) to another MSC (MSC-B);

b) the Subsequent Inter-MSC Handover procedure, where the MS is handed over from MSC-B to a third MSC (MSC-B');

c) the Subsequent Inter-MSC handback, where the MS is handed back from MSC-B to MSC-A.

In both cases i) and ii) the same procedures as defined in the GSM 08.08 and the GSM 04.08 shall be used on the A-interface and on the Radio Interface, respectively. In case ii) the handover procedures shall transport the A-interface messages between MSC-A and MSC-B described in the Mobile Application Part (MAP), GSM 09.02. The split in functionality between the BSS and MSC is described in the GSM 08 series of Technical Specifications.

The interworking between the GSM 09.02 protocol and the GSM 08.08 protocol is described in the GSM 09.10 Technical Specification.

Handovers which take place on the same MSC are termed Intra-MSC handovers; this includes both Inter-BSS and Intra-BSS handovers.

The present document also covers the requirements for handover in ongoing voice group calls, directed retry and the handover without a circuit connection between MSC-A and MSC-B. The present document does not consider the case of handovers between radio channels on the same BSS (Intra-BSS handover) or the handover of packet radio services.

For voice broadcast calls, the speaker uses normal point-to-point handover procedures, whilst the listeners use idle mode cell reselection procedures, as for the voice group call listeners.

Inter-MSC hand-over imposes a few limitations on the system. After inter-MSC hand-over:

- call re-establishment is not supported.

The list of GSM 08.08 features supported during and after Inter-MSC handover is given in GSM 09.08.

In the Inter-MSC handover case, the interworking between a Phase 1 BSSMAP protocol possibly used by one MSC and the Phase 2 BSSMAP protocol used in the Phase 2 MAP protocol on the E-interface is performed by this MSC.

NOTE: The message primitive names used in the SDL diagrams and message flows in the present document do not represent the actual messages specified in the GSM 04 and GSM 08 series or the services specified in the GSM 09 series of technical specifications. The primitive names are only intended to be indicative of their use in the present document.